Quad flat no lead (QFN) integrated circuit (IC) package having a modified paddle and method for designing the package

Abstract

A QFN IC package is provided that has all of the advantages of the typical QFN IC package, but in addition, has a paddle that is configured to facilitate trace routing and/or via placement on the PWB or PCB on which the IC package is mounted. By configuring the paddle as necessary or desired in order to facilitate routing and/or via placement, the overall size of the PWB or PCB can be reduced without sacrificing the thermal or electrical performance advantages that the paddle provides. In addition, the reduction in the overall size of the PWB or PCB results in reduced cost.

1. A Quad Flat No Lead (QFN) integrated circuit (IC) package comprising:
 a leadframe comprising one or more electrically conductive lands and an electrically and thermally conductive paddle, the paddle having an exposed portion and an unexposed portion, the unexposed portion of the paddle having a top surface having a width in an X dimension and a length in a Y dimension, the top surface of the unexposed portion having an area equal to the width of the top surface of the unexposed portion times the length of the top surface of the unexposed portion, the bottom surface of the exposed portion of the paddle for attachment to a printed circuit or printed wiring board and having a width in the X dimension and a length in the Y dimension, the bottom surface of the exposed portion having an area equal to the width of the bottom surface of the exposed portion of the paddle times the length of the bottom surface of the exposed portion; 
 a die having a bottom surface attached to the top surface of the unexposed portion of the paddle, the die having a width in an X dimension and a length in a Y dimension, the bottom surface of the die having an area equal to the width of the die times the length of the die; 
 an encasement that at least partially encases the leadframe and the die, the lands having ends that are exposed through the encasement and that are substantially flush with the encasement, the bottom surface of the exposed portion of the paddle being exposed through the encasement and substantially flush with the encasement; and 
 wherein the area of the bottom surface of the exposed portion of the paddle is smaller than the area of the top surface of the unexposed portion of the paddle, the area of the bottom surface of the exposed portion of the paddle is also smaller than the area of the bottom surface of the die, the bottom surface of the exposed portion of the paddle is substantially flat and has a shape which is substantially the same as the bottom surface of the die, and the bottom surface of the exposed portion of the paddle is generally square in shape and has a width and length that are equal and range from about 0.5 mm to 3.5 mm, and wherein the area of the bottom surface of the exposed portion ranges from about 0.25 millimeters squared (mm 2 ) to about 12.25 mm 2 . 
 
     
     
       2. The QFN IC package of  claim 1 , wherein the bottom surface of the exposed portion has a length and width of about 1.85 mm, the area of the bottom surface of the exposed portion being about 3.42 mm 2 . 
     
     
       3. The QFN IC package of  claim 1 , wherein the top surface of the unexposed portion of the paddle is substantially flat and has an area that is larger than the area of the bottom surface of the die. 
     
     
       4. The QFN IC package of  claim 1 , wherein the exposed portion of the paddle comprises at least two pillars that are generally cylindrical in shape, each pillar having a top surface that coincides with a bottom surface of the unexposed portion of the paddle, the pillars having respective bottom surfaces that make up the bottom surface of the exposed portion of the paddle. 
     
     
       5. A Quad Flat No Lead (QFN) integrated circuit (IC) package comprising:
 a leadframe comprising one or more electrically conductive lands and an electrically and thermally conductive paddle, the paddle having an exposed portion and an unexposed portion, the unexposed portion of the paddle having a top surface having a width in an X dimension and a length in a Y dimension, the top surface of the unexposed portion having an area equal to the width of the top surface of the unexposed portion times the length of the top surface of the unexposed portion, the bottom surface of the exposed portion of the paddle for attachment to a printed circuit or printed wiring board and having a width in the X dimension and a length in the Y dimension, the bottom surface of the exposed portion having an area equal to the width of the bottom surface of the exposed portion of the paddle times the length of the bottom surface of the exposed portion; 
 a die having a bottom surface attached to the top surface of the unexposed portion of the paddle, the die having a width in an X dimension and a length in a Y dimension, the bottom surface of the die having an area equal to the width of the die times the length of the die; 
 an encasement that at least partially encases the leadframe and the die the lands having ends that are exposed through the encasement and that are substantially flush with the encasement, the bottom surface of the exposed portion of the paddle being exposed through the encasement and substantially flush with the encasement; and 
 wherein the area of the bottom surface of the exposed portion of the paddle is smaller than the area of the top surface of the unexposed portion of the paddle, the area of the bottom surface of the exposed portion of the paddle is also smaller than the area of the bottom surface of the die, the bottom surface of the exposed portion of the paddle is substantially flat and has a shape which is substantially the same as the bottom surface of the die, and the die has a thickness in a Z dimension that is less than or equal to about 7 mils, a mil being equal to 10 −3  inches. 
 
     
     
       6. A leadframe for Quad Flat No Lead (QFN) integrated circuit (IC) package, the leadframe comprising:
 one or more electrically conductive lands; and 
 an electrically and thermally conductive paddle, the paddle having an exposed bottom surface for attachment to a printed circuit or printed wiring board and having an unexposed top surface for attachment to a bottom surface of a die, the exposed bottom surface of the paddle being smaller in area than the unexposed top surface of the paddle, the exposed bottom surface of the paddle also being smaller in the area than the bottom surface of the die, the exposed bottom surface of the paddle is substantially flat and has a shape which is substantially the same as the bottom surface of the die, wherein the exposed bottom surface of the paddle is generally square in shape and has a width and length that are less than about 3.5 mm and an area of less than 12.25 mm 2 . 
 
     
     
       7. The leadframe of  claim 6 , wherein the exposed bottom surface of the paddle is substantially flat. 
     
     
       8. A method for creating a leadframe for Quad Flat No Lead (QFN) integrated circuit (IC) package, the method comprising:
 selecting a leadframe design having a paddle with a bottom exposed surface for attachment to a printed circuit board or printed wiring board that is smaller in area than a top surface of an unexposed portion of the paddle; and 
 attaching the top surface of the unexposed portion of the paddle to a bottom surface of a die, the bottom exposed surface of the paddle being smaller in the area than the bottom surface of the die, the bottom exposed surface of the paddle is substantially flat, has a shape which is substantially the same as the bottom surface of the die, and has a width and length that are less than 3.5 mm. 
 
     
     
       9. The method of  claim 8 , wherein the leadframe design is selected based on a design of a printed circuit board or printed wiring board to which the bottom exposed surface is to be attached. 
     
     
       10. The method of  claim 8 , wherein the die has a thickness is a Z dimension that is less than approximately 7 mils, a mil being equal to 10 −3  inches.